Introduction

Marine animals experience disease outbreaks similar to human virus epidemics. In the last thirty years such outbreaks have become more frequent. Disease events in animal populations are called epizootics. Severe epizootics can wipe out whole populations (or the majority of individuals), which can lead to an imbalance in the ecosystem as a predator or prey species is wiped out of the area.

Environmental characteristics can create an opportunity for epizootics through the appropriate temperature, salinity or pH for a given pathogen. Climate change poses increased threats, as some diseases may be able to take advantage of changing conditions and increase in frequency or intensity of infection.

The researchers explored the increase of disease outbreaks in the green sea urchin (Strongylocentrotus droebachiensis) on the Atlantic coast of Nova Scotia, Canada. The urchins fall victim to an amoeba pathogen (Paramoeba invadens). This amoeba alters the urchin population- its presence or absence determining urchin numbers. Green urchins graze heavily on kelp beds and thus regulate the kelp population. In the 1970s urchins became so abundant that coast line covered in kelp beds turned to barren, rocky habitat. In the 1980s an amoeba hit the scene and depleted urchin numbers until kelp beds reestablished themselves. Recently an amoeba has returned to the area and is causing mass die-offs of urchins. Once again the kelp-urchin balance is up for grabs.

Amoeba outbreaks occur in late summer and early fall when temperatures are highest. Traditionally low temperatures have knocked out amoeba populations during winter months. It had been thought that large storms blew in new populations of amoeba thus leading to disease outbreak. But, we are now seeing disease outbreaks without a large storm preceding it- implying the amoebae are overwintering (staying in the area, rather than being blown in with a storm). This paper investigates the ability of the amoeba to survive at low temperatures they would face during a Nova Scotia winter.

Samples of the amoeba pathogen were collected from sea urchins during a disease outbreak. The researchers monitored survival and growth rate of the amoeba at 0.5, 2.0, and 3.5°C. The amoebae were fed a bacterial diet of E. coli. The experiments explore two questions that help explain how amoebae could stay alive over the winter and reemerge the following year.

Can amoebae tolerate colder temperatures than they could 3 decades ago? The amoebae were raised in the low temperature treatments or at a high temperature that was known to allow for amoeba-induced disease. The experiment ran for up to 97 days and the amoeba concentration was monitored throughout.

How well do amoebae recover after living in low temperatures? After the first phase of experiments some amoebae were returned to high temperatures after 25, 32, 62 or 96 days and survival was monitored to determine the lowest temperature from which the amoeba population could recover. Too long at low temperatures will kill off amoebae, but we don’t know what temperature or duration is too much.

Results

The experiments showed the amoebae died at the lowest temperatures after one month and showed no potential for recovery even after a shorter period of time. Previously spherical cells were not seen in the samples but began to appear at low temperatures and were correlated with higher mortality rates. After three months at 3.5°C the amoeba could recover when placed in warmer temperatures, suggesting the thermal threshold is somewhat below this temperature- between 2 -3 °C.

The researchers compared the results of their experiments with the actual temperatures of the ocean to determine if disease outbreaks were linked to the overwintering survival of the amoeba. Only two years in the last 35 years showed disease outbreaks that were not immediately following a storm. Both of these years the mean sea surface temperature in winter was above 2.5°C.

Change over time in concentration of cells of Paramoeba invadens that are amoeboid (active), degenerating (dying) and spherical (mL− 1) in monoxenic culture at A) 0.5 °C, B) 2.0 °C and C) 3.5 °C in the growth experiment.

Maximum number of consecutive days (d) with mean sea temperatures below 6 low-temperature thresholds from 1.5 °C to 4.0 °C (at 0.5 °C increments) in the past decade (2005–2014) for the period of minimum annual sea temperatures (February–March). Temperature data are from a thermograph at 8 m depth at a site (The Lodge) in St. Margarets Bay. Graphics indicate the presence or absence of a sea urchin disease outbreak along the coast of Nova Scotia in the following summer–fall, and whether a disease outbreak was preceded by a hurricane or strong storm.

Significance

As sea surface temperature continues to rise disease pathogens will continue to find new opportunities to cause outbreaks. Understanding how disease outbreaks may be affected by temperature changes allows for better-informed management and disaster response planning.

What other important marine predators, such as sea urchins, shape their community? How might other aspects of climate change (ocean acidification, increased storms) effect the relationship between sea urchins, kelp and amoeba pathogens?